1. Field of Invention
The present invention relates to a derrick for drilling and maintenance of oil and/or gas wells, especially intended for installation on a marine platform.
2. Related Art
The derrick structure used today for this purpose comprises an elevator drawwork driven by several direct current motors. The electrical power is transmitted via expensive and complicated, controlled rectifier systems. The elevator drawwork itself consists of several expensive and heavy components. A powerful, chain-driven winch is used which obtains power via large transmission gears, and the winch drives a drum that hoists the drilling line. The drilling line passes over a crown block and through a traveling block before it is fastened to a so-called "dead anchor". The drilling line must be replaced regularly. All of the equipment used today is very expensive. The equipment is also exceedingly heavy and this requires that the derrick itself be dimensioned accordingly, so that the derrick is tall, massive, heavy and expensive. Today's equipment is not adapted for accommodating equipment for cleaning the production pipes. Both the costs and the risks involved when performing this cleaning operation are very high. The only available equipment is extremely expensive, time-consuming to assemble and use, and unsatisfactory from a safety point of view. Moreover, it requires a huge power source. The power lost through the chain drives and all the blocks over which the drilling line must pass corresponds to almost half of the total power consumption. On floating rigs, heave compensators (assemblies to compensate for vertical movements due to wave surge) must be used during the drilling operations and when installing a blow-out preventer in the well head. A heave compensator today costs about NOK 10 million. Another drawback of the traditional elevator drawwork is its very high noise level.
Swiss Pat. No. 495,278 generally discloses a compartment-type elevator which carries hydraulic motors provided with gears that engage with vertical racks and drive the elevator along the racks.
U.S. Pat. No. 2,514,498 discloses an elevator for use in mine shafts utilizing, inter alia, stationary, vertical racks that mesh with gears driven from the elevator's load-bearing member.
British Pat. No. 1,431,759 describes a hoist for lifting a load from a reference point under conditions in which the reference point and the hoisting apparatus may move vertically in relation to each other, as might be the case when a heavy load is being hoisted from a pitching ship deck at sea. This specification suggests the use of a heave compensator to compensate for wave surge.